Filmmaker Matthew Heineman floored audiences two years ago with his third feature film Cartel Land, an in-the-foxhole, vérité tale of the ongoing drug problem along the U.S.-Mexican border. It earned him an Academy Award nomination and won him an Emmy, after dominating at the 2015 Sundance Film Festival with a directing award and a special jury prize for cinematography.

This January, he was back in the snowy peaks of Park City with City of Ghosts, a powerful cinematic experience that follows the journey of "Raqqa is Being Slaughtered Silently" (RBSS) over a year of their lives. RBSS is a small group of anonymous activists and unwitting journalists who banded together after their city in Syria was taken over by ISIS in 2014 to get their stories out to the world undercover, on the run and in exile, risking their lives to stand up against one of the great evils in the world today.

Documentary Magazine was able to speak with Heineman as he hopped around the world, from one destination to another, promoting his film and the important work of RBSS, bringing the tragedy of Syria to the forefront of people's minds with heartfelt filmmaking. Here, he tells us what it was like observing these truth-seekers.

What was it like segueing from Cartel Land to City of Ghosts - or making the latter while still promoting the former?

Matthew Heineman: As I was traveling with my last film [through the festival circuit], the plight of Syria was becoming a near daily part of the news cycle. I almost obsessively began reading about what was happening with the so-called Islamic State. In the fall of 2015, I read an article about RBSS, a group of citizen activists who came together to document the atrocities that ISIS was committing in their hometown – dubbed the capital of the Islamic State. I was moved by the sacrifices that they had endured as a group, and I knew their story could provide an intimate, character-driven window into life under ISIS.

I made contact with members of RBSS, attempted to gain their trust and soon began filming with them. I knew almost immediately that I wanted the spine of the story to be deeply personal vérité footage, captured as the activists escaped Syria after the assassination of several members by ISIS. I followed them in Turkey and then eventually to Europe as ISIS continued to threaten them.

At the beginning, there was certainly a fair amount of juggling between promoting Cartel Land and filming City of Ghosts, but ultimately I was grateful to jump back into the creative process while making the press circuit rounds [with Cartel Land].

What were the dangers and challenges of making this film?

This was an incredibly difficult film to make for a variety of reasons. The safety of the members was always paramount, so we communicated with them through encrypted means and were very careful how, where and when we filmed with them.

Much of City of Ghosts takes place as members of the group were on the run escaping Syria and ultimately landing in Europe. Because of this, and the nature of their work, much of the drama takes place in safe houses and behind computers and cell phones. It was challenging to make these situations – which could be quite static – dynamic. Over the months that we were filming, I tried to find these dynamic moments that would, ultimately, make the film more cinematic.

Were you ever in danger?

Honestly, any danger that I put myself in by making this film truly pales in comparison to the danger the subjects are in. They have lost members of their group, they have lost family, and they continue to be threatened by ISIS for standing up against them. In addition, their colleagues who are still inside Raqqa reporting and sending out information are in the gravest danger of all.

What precautions did you take?

We took many precautions throughout the production process, but also in post. In addition to our communication with members of RBSS, we all communicated through encrypted means, used encrypted drives and never shared links through the post process. We consulted a number of experts and other filmmakers like Laura Poitras [Citizenfour, The Oath] and her team, who were very helpful in guiding us through the world of cyber security. And our executive producer Alex Gibney - known for his prolific work as a documentarian with films like Taxi to the Dark Side and Going Clear: Scientology and the Prison of Belief - who has a great deal of experience navigating these sorts of situations.

Of course your subjects were in danger - this is talked about throughout the whole film - but how did that affect you, as a purveyor of their world and knowing you'd go home to a safe environment?

City of Ghosts is a testament to the bravery of the men and women of RBSS, who have risked everything in pursuit of the truth, in pursuit of exposing evil. I hope the film can amplify their efforts in making sure that their friends, their families and their city of Raqqa is not being slaughtered silently.

Spending time with members of the group over the past year changed me profoundly. I have a much stronger connection to Syria and still read voraciously about the daily horrors that occur there. The individuals that I filmed with in exile and their colleagues inside Raqqa, the capital of the Islamic State, are in grave danger. As with Cartel Land, it was disorienting to edit such horrifying images within the comfort and safety of New York City. For me, the key is to never become complacent with that horror or that danger and to try and inject it into every frame of the film, whether consciously or unconsciously.

How did you find your subjects?

It was actually quite fortuitous how I came upon the story of "Raqqa Is Being Slaughtered Silently." I with my friend and fellow filmmaker Jimmy Chin, and he was telling me about a series of articles he had read in the New Yorker's daily e-blast. I subscribe to the magazine, but decided to sign up for this e-blast that day. And within a few weeks I received an article about RBSS by David Remnick. Through the Committee to Protect Journalists, I reached out to RBSS that night, and I was filming with them a week later.

Did you not need to wait for funding or producers or any of the things that most filmmakers need to do before going into production?

As someone who not only directs, but also shoots and edits as well, I don't always have to wait for funding to start on a project because I don't have to go hire a DP, a sound person and other crew before shooting. This is what I did with City of Ghosts. For other directors who don't shoot or do sound, it takes a certain amount of money to gather those resources. I guess I'm lucky in that sense.

Did you stay with RBSS the entire time while they were in exile?

I filmed with members of the group on and off for about nine months in Turkey and Europe as they were on the run from ISIS and ultimately finding asylum.

Assuming that you don’t know Arabic, how did you follow the story, or pick up on key aspects of the story?

For Cartel Land I spoke marginally conversational Spanish. For City of Ghosts I really didn't speak any Arabic. It obviously made it more difficult, but I also found it to be an advantage while shooting. It allowed me to focus on the emotion of the scene as opposed to just chasing dialogue. Syrians are quite expressive in how they communicate – they wear so much of what they say on their face – so I found that interesting to try and follow, even if I didn’t totally understand the words that were being expressed. Nonetheless, during breaks and before and after shoots, I always checked in with some of the group members who spoke English and they would explain what was happening, or what had just happened, so that I was always in the loop.

City of Ghosts opens in theaters July 14 through IFC Films, Amazon Stdios and A&E Indie Films.

Valentina Valentini is a freelance journalist traveling the world in search of her next story. She covers entertainment, travel, food, culture and people for Vanity Fair, BBC Travel, Marie Claire, and many more. Follow her adventure at @tiniv.